Fingerprints, palm prints and foot prints are impressions of the friction ridge skin present on the palm side of the hand and soles of the feet. When a person touches, grabs, or walks barefoot on a surface an impression of the friction ridge skin may be left behind. These unintentional impressions are called latent prints.
The importance of latent print evidence is its ability to identify an individual. Latent prints can be identified to a single person because the friction ridge skin possesses two key properties: permanence and uniqueness. The friction ridge skin forms before birth and does not change naturally, except to grow larger, until after death. With the exception of injury, a person has the same fingerprints, palm prints, and foot prints from early gestation until decomposition after death. Additionally, the friction ridge skin has unique characteristics which allow even small latent prints to be identified to a single person, in some instances.
Latent prints left on a surface can be visualized through a variety of chemical and physical development techniques. Once visible, the latent print can be photographed and compared to the known inked fingerprints (or palm prints or foot prints) of an individual. Through careful analysis and comparison of the friction ridge skin characteristics in both the unknown latent print and the known inked print, a person can be identified, or eliminated, as having made the print.
If the origin of the latent fingerprint is unknown, it may be searched through the Western Identification Network (WIN). WIN is a computer database which stores the images of inked fingerprints from several Western states and provides links to the fingerprint databases of even more states. When an unknown latent fingerprint is searched through the database, a list of possible candidates is generated and reviewed by an analyst.
How are people identified by their fingerprints?
Friction skin structure is used to identify impressions of fingers, palms, and feet as having been made by a particular person. This type of skin is made up of ridges which are both permanent and unique, barring advanced injury. The size, shape and relative positions of structures of friction skin features are compared. With sufficient quality and quantity of these features in correspondence, and identification is made. There is no set minimum number of features that must be in correspondence, it is through experience, training, and the process of verification that latent print identification standards are determined.
Do people always leave fingerprints when they touch something?
No. Many factors determine whether or not a latent print will be of sufficient quality for comparison. These factors include; the condition of the surface, the presence of a substance on the friction skin, environmental and physical conditions the latent is subjected to after deposition, and the nature of the initial touch. For example, if the surface is very dirty, or the hands are very clean, or the surface is exposed to rain, or the finger is wiped across the surface, the print may be non-existent or wiped or washed away. Conversely, if the surface is dry and smooth, the hands are very sweaty, the initial touch is direct, and the latent is protected until examined, the latent print may contain many clear and visible ridges for comparison.
How can 'invisible' fingerprints, made up of only sweat, be seen?
The term 'latent' means invisible. Technically, prints which are visible, for instance a bloody fingerprint impression, are referred to as 'patent' prints. Latent impressions, made up only of sweat and/or oil, are very difficult to see. Through the use of crime scene lighting techniques, photography, and the application of chemicals, these impressions are enhanced so that the details of the print can be viewed. Chemicals used range from the standard black powder to specialized chemicals which react with amino acids in sweat, are specific to the sticky side of tape, or are fluorescent when exposed to particular wavelengths of light.